There’s a couple of things that’s implied by democrats saying Biden shouldn’t run next year.
The most obvious implication is that Biden has been a disaster. That’s the kind interpretation.
Somewhat less obvious is the implication of a total lack of understanding of why Biden is a disaster.
The truth of the matter is, Biden is not a screw up. That’s far too easy, making Biden a scapegoat at need.
The country is not in trouble because Biden screwed up the implementation of Democratic party policy, but rather that he perfectly implemented Democrat party policy. That’s something to rank and file Democrats understand but won’t admit to, even the true believers. And THAT point, my friends, is the most frightening thing of all.